Silicon nitride powder is an interesting raw material for the production of ceramic materials which are to be exposed to severe mechanical and thermal conditions.
Silicon nitride powder used as raw material for the production of ceramic materials for high temperature uses are required to meet high standards of chemical purity and contain a high proportion of .alpha.-Si.sub.3 N.sub.4.
If ceramic Si.sub.3 N.sub.4 materials contain a high proportion of metallic or non-metallic impurities such as oxygen or carbon, their high temperature properties are impaired.
A high proportion of .alpha.-Si.sub.3 N.sub.4 to .beta.-Si.sub.3 N.sub.4 in the raw materials is advantageous for the sintering properties and promotes the formation of a dense microstructure in the finished material.
The following are the most important processes for the production of silicon nitride powder;
(1) direct nitridation of silicon with nitrogen; PA0 (2) the reaction of SiCl.sub.4 with ammonia; PA0 (3) carbothermal nitridation of SiO.sub.2 in the presence of carbon.
Among these processes, carbothermal nitridation of SiO.sub.2 (3) is of particular interest for large scale industrial production.
Direct nitridation of silicon with nitrogen (1) is an exothermic reaction which requires accurate control of the reaction process if powders containing a high proportion of .alpha.-Si.sub.3 N.sub.4 are to be obtained.
The process employing the reaction of SiCl.sub.4 with ammonia (2) has the disadvantage that it requires the use of expensive raw materials, such as SiCl.sub.4, and that substantial quantities of by-products, such as HCl and NH.sub.4 Cl, are formed in addition to the silicon nitride. This process is therefore less suitable for the production of a silicon nitride powder at an economical cost.
The process of carbothermal nitridation (3), on the other hand, has the advantage of being particularly interesting for large scale industrial production of Si.sub.3 N.sub.4 because the reaction is simple and the SiO.sub.2 raw material is inexpensive and readily available.
If the process (3) is to be employed on an industrial scale, however, it has the disadvantage that reactions between solids, in this case SiO.sub.2 and carbon, and gases depend to a large extent on the reaction velocity of diffusion processes. The diffusion of gas is considerably impeded if the reaction mixtures form a high dumping height, and the reaction conditions are then adversely affected. Carbothermal nitridation therefore requires long reaction times and high reaction temperatures if adequate nitridation rates are to be achieved.
Various processes have been proposed to overcome these disadvantages.
Japanese Patent Application 76/28599 describes a method of accelerating the carbothermal nitridation reaction by the addition of oxides of the elements iron, aluminium, calcium and magnesium to a mixture of amorphous SiO.sub.2 and carbon.
Also European Patent Application 80,050 describes a method of increasing the .alpha.-Si.sub.3 N.sub.4 content by the addition of compounds of alkaline earth metals or transition elements.
These processes, however, are not suitable for the production of highly pure silicon nitride powder because the compounds added severely contaminate the powder obtained, especially with elements such as calcium, iron or aluminium which impair the high temperatures strength of Si.sub.3 N.sub.4 materials.
It was thus an object of the present invention to provide a process for the production of silicon nitride which would not have the disadvantages described above.